1. Field of the Invention
The present invention relates to a defect inspection system for photomasks used in transferring reduction patterns of electronic circuit and/or lines or interconnections onto a crystalline substrate, and more particularly, to a defect inspection system for photomasks, in particular phase shift masks for use in reduction pattern transfer on the order of a submicron.
2. Description of the Related Art
In recent technology very large scale integrated (VLSIA) circuits integrate from hundreds of thousands to several million transistors and interconnections of submicron order on the surface of a single chip silicon crystalline substrate. These fine transistors and lines are patterned by a photolithography process in which a pattern of submicron order is formed, reduced, and then transferred on (a smaller scale [one-fourth to one-tenth]) onto a silicon substrate with a photoresist layer applied thereon.
More specifically, the photolithographic process comprises the steps of arranging transparent portions and opaque portions formed with Cr film and the like on a photoresist layer applied on a crystalline substrate (for example, a silicon crystalline substrate), and irradiating illumination light onto a photoresist mask of a certain pattern so as to transfer a mask pattern (that is, a pattern reduced on a scale of 1/4 to 1/10 from an original pattern by means of a reducting projection exposure device). In the current mass-production, 1 megabit and 4 megabit DRAM's (abbreviation of Dynamic Random Access Memory), are patterned with lines having a minimum line width of 1.2 .mu.m and 0.8 .mu.m, respectively. In the photolithography to form patterns having such linewidths, the illumination light irradiated onto photomasks employed, in most cases, is a g-line (a bright emission line having a wavelength of 436 nm) which is radiated from an extra-high pressure mercury lamp. However, recently an i-line (bright emission line having a wavelength of 365 nm) emitted from the same mercury lamp has also been used.
The anticipated minimum line widths for use in 16 megabit DRAM's and 64 megabit DRAM's (which are expected to be fabricated in the future) are estimated to be 0.6 to 0.5 .mu.m and 0.4 to 0.3 .mu.m, respectively. In order to achieve the mass production of these semiconductor devices, it is first necessary to form photomasks having such minimum line widths, and this requires increased photo-lithography in its resolution. To deal with this requirement, or in order to improve the resolution by using light of shorter wavelengths, not only has there been use of i-line (in place of g-line) investigated and discussed but also employment of a krypton-fluorine exciter laser having a shorter wavelength, e.g. of 248 nm.
The photolithography using the above-discussed conventional photomasks, however, has a following drawback. That is, a pattern image formed by a transparent portion of a photomask on the photoresist layer of a crystalline substrate as a mask pattern is difficult to separate from that by a neighbor transparent portion, due to the interference between the diffracted light from the the edge of the first transparent portion and that from the edge of the neighbor transparent portion. A resolution of a step and repeat photolithographic system with demagnification is determined in theory by a numerical aperture NA and the wavelength .lambda. of the illumination light. However, for the reason just mentioned above, the resolution obtained in practice is by far inferior to the theoretical value.
A photomask for overcoming the difficulty stated above is proposed in Japanese Patent Application Laid-Open No. Sho-57-62052 (1982), laid open on Apr. 14, 1982, invented by Masato Shibuya, the title of which is "A Photomask to be Projected by a Transmissive Illumination."
In that invention, i.e. "A Photomask to be Projected by a transmissive Illumination", a photoresist mask is proposed which is constructed such that a predetermined pattern with transparent portions and opaque portions, and at least one of transparent portions between which an opaque portion is located is provided with a phase shift member (for example a 180.degree. phase shifter) so as to cause a phase difference between the nearest neighboring transparent portions.
Another report is "Improving Resolution in Photolithography with a Phase-Shifting Mask" by M.D.Levenson et al. appears in "IEEE Transactions on Electron Devices" Vol. ED-29 No. 12, p.p 1828-1846, 1982"published by Institute of Electrical and Electronics Engineers in U.S.A. This article reported in effect that use of a photomask having a similar phase member (i.e. 180.degree. phase-shifting member) with that disclosed by the aforementioned Japanese Patent Application Laid-Open No.sho-57-62052 (1982) aforementioned, increases resolution and depth of focus in photolithography.
Further, a study on submicron resist exposures and simulations of transfer image was made by M.D. Levenson et al. and reported in "The Phase Shifting Mask II: Imaging Simulations and Submicron Resist Exposures" in IEEE Transactions on Electron Devices, Vol. ED-31, No. 6(1984) pp. 753-763.
In accordance with the photolithographic technique using a phase shift mask as stated above, it is possible to obtain a high resolution transfer image having a minimum linewidth in the order of submicron. Even if, however, there were a photomask capable of providing a transfer image with a high resolution, if the mask in itself has defects (such as, for example, a protrusion, or cracked portion, at an edge of or inside, a transparent portion of a phase shift member) the mask could not be used for the photolithography. Accordingly, it is necessary to control the photoresist mask before operation such that the presence and absence of defects is inspected on the phase shift mask to be used, so that the mask can be discarded if it there is a defect.
As regards detection of defects in a phase-shift mask, there is a report of James N. Wiley et al., entitled "Phase-Shift Mask Pattern Accuracy Requirements and Inspection Technology", included in a lecture preparation text "Integrated Circuit Metrology, Inspection, and Process Control V", William H. Arnold, Editor, Proc SPIE 1464, pp. 346-355 (1991)" published by The International Society for Optical Engineering (SPIE for abbreviated name). According to this article, the evaluation was carried out by KLA 219 HR-PS prototype die-to-die photomask inspection system. The inspection system mentioned immediately above is constructed as shown in FIG. 1. Specifically, illumination light emitted from a mercury lamp 1 is irradiated onto a phase shift mask 2 formed with a certain mask pattern having, for example, two neighboring dies A, B. The transmitted light carrying the image patterns of these two dies A, B is individually introduced to respective optical systems (3-1) and (3-2) for projection. The thus transmitted light through the optical systems, passing through an image acquisition device 4 (including a charge coupled device and the like), is then filtered by noise removal filter 5 to be provided to an alignment device 6. Two image patterns of dies A, B are overlapped or superposed on one another in the alignment device 6 so as to detect (by means of defect detection section 7) the presence of defects in the transparent area on the phase shift mask 2.
The phase shift mask 2a examined by the KLA219HR-PS prototype die-to-die photomask inspection system shown in FIG. 1, includes (as shown in FIGS. 2A and 2B, for example), on a transparent quartz wafer 2 a resist layer (2-3) applied thereon, opaque Cr films (2-2), (2-2) adhered apart from each other by a certain distance, an isolated Cr film (2-4), a self-aligned phase mask which is formed with a phase shift layer (2-5) for sharping the contour of the Cr film (2-4), and has a larger area than that of the Cr film (2-4).
The inspection system shown in FIG. 1 comprises a light source for exposure illumination (e.g. a mercury lamp) 1; projection optical systems (3-1), (3-2) for projecting and image-forming the illumination "light transmitted through respective dies A, B on the phase shift mask onto an image acquisition section 4; a noise removal filter 5 (for removing noise from the data signals of the images representing the dies A, B, and projected on the image acquisition section 4); an alignment device 6 for receiving the image data signals of the dies A, B (noise-freed by means of the noise removal filter 5, so as to superpose the image patterns of dies A and B); and a defect detection section 7 for receiving the image pattern data overlapped in the alignment 6 to be stored in advance and comparing the pattern data of a phase shift mask under test, so as to detect defects on the phase shift mask under test.
In this inspection system, the same detection operations as in the examination on two dies A and B of the phase-shift mask under test will be repeated successively with respect to other pairs of dies to complete defect inspection for every die on the entire phase shift mask.
In the inspection system described above, since the illumination light (a mercury lamp in FIG. 1) generally consists of polychromatic light, scattering at an edge of a transparent portion in a phase shift mask is not constant. As a result it is impossible to obtain an accurate detection sensitivity to defects. The inspection system of this type has a defect inspection capability or resolution of about 400 nm, which is inferior to the defect inspection capability for metal masks used in the same semiconductor manufacturing process. Here, the metal mask indicates an exposure mask having a predetermined pattern of opaque metal films arranged on the transparent quartz wafer by separating one another with providing light-transmissive clearances having a certain width.
From a manufacturing view point, however, a defect on a phase-shift mask is more likely to be transferred than that on a metal mask. For this reason, the criteria for the defect inspection for phase-shift masks is more stringent than for that of metal masks.
Moreover, in the inspection system shown in FIG. 1 above, a defect is detected by means of scattered light, therefore, defects along edges of transparent portions alone can be detected, but those inside, or enclosed by, a transparent portion are beyond detection.,
Since defects of phase shift masks may possibly appear not only at the edges of transparent portions but also thereinside, development of a phase shift mask defect inspection system capable of detecting defects in the entire phase-shift mask would be highly desirable.
In addition, in the inspection system of FIG. 1 described above, two dies under test are examined alternately for comparison, so that it takes a long time to examine the patterns. Accordingly, a phase shift mask defect inspection system which can examine both dies simultaneously has long been sought.